In order to effectively utilize resources used for processing image data, compressing and restoring methods of image data are widely used.
As one of the compressing and restoring methods of image data, a method is known in which all the image elements included in image data are compressed and restored on the basis of one algorithm. In the conventional method of compressing and restoring image data, image data including various kinds of image elements, such as types, handwritten characters, handwritten drawings, free micro dots, tables, illustrations, graphics, tints and photographs are compressed and restored on the basis of the one algorithm.
However, the conventional method of compressing and restoring the image data is difficult to suppress deterioration in a picture quality caused by the compression of the image data for all the image elements included in the image data. For example, when an algorithm which suppresses deterioration of the picture quality in compressing and restoring an image data generated from a photograph is used to compress and restore other image data generated from characters and lines, the edges of the characters and the lines are not clearly restored. Conversely, when another algorithm which suppresses deterioration of the picture quality in compressing and restoring image data generated from characters and lines is used to compress and restore the image data generated from photographs, the image of the photograph tend to be distorted.
A compressing and restoring method of the image data is desirably designed so as to suppress the deterioration in the picture quality caused by the compression of the image data.
Moreover, the conventional method of compressing and restoring the image data is difficult to attain both the improvement of a compression rate and the suppression of the deterioration in the image data caused by the compression of the image data with respect to all the image elements. If image data generated from the image elements such as types, handwritten characters, handwritten drawings, tables and illustrations is compressed and restored on the basis of the algorithm effective for image data generated from photographs, edges of the image elements are disgracefully restored. Furthermore, a focus of the restored picture becomes loose. On the other hand, use of an effective method for compressing the line pictures and the like deteriorates the picture qualities of graphics, tints, photographs and the like. Moreover, a data amount of the compressed data is increased.
The compressing and restoring method of the image data is desirably designed to increase the compression rate and to suppress deterioration of the picture quality caused by the compression of the image data.
Also, operations of compressing and restoring image data generated from pictures drawn on printed matters are generally carried out. As shown in FIG. 41, a picture drawn on a colored printed matter is provided with pixels 501. Each of the pixels 501 is composed of a blue dot 502a, a red dot 502b, a yellow dot 502c and a black dot 502d. The blue dot 502a is a dot provided by a blue screen (a C screen). The red dot 502b is a dot provided by a red screen (an M screen). The yellow dot 502c is a dot provided by a yellow screen (a Y screen). The black dot 502d is a dot provided by a black component (a K screen). The arrangement of the blue dot 502a, the red dot 502b, the yellow dot 502c and the black dot 502d is not limited to that shown in FIG. 41. The blue dot 502a, the red dot 502b, the yellow dot 502c and the black dot 502d are collectively referred to as a dot 502. The dot 502 may be square as shown in FIG. 42A, and may be differently shaped, for example, circular as shown in FIG. 42B.
The dots 502 included in one screen are regularly arranged on screen lines 503 in accordance with a print rule, as shown in FIG. 43. The screen lines 503 and an X-axis cross each other at an angle defined by the print rule. The angle is different for each of the screens. The screen lines 503 are arranged in parallel to each other at an equal interval. A screen ruling is defined as being 1/ds, where ds is the interval between the screen lines 503. When a line segment of a unit length, typically one inch, in a direction vertical to the screen line 503 is considered, the screen ruling implies the number of the screen lines 503 crossing the line segment.
The dot 502 is arranged such that the center thereof is located on the screen line 503. Here, when the dot 502 is square, the center of the dot 502 implies a point at which the diagonals thereof cross each other. When the dot 502 is formed in a circle, the center of the dot 502 implies a center of the circle.
An area of each dot 502 indicates a graduation. As the area of the dot 502 is larger, an eye of a human recognizes that a concentration of a position of the dot 502 is higher.
The picture composed of the dots having the arrangements and the shapes as mentioned above originally has a large redundancy.
However, the conventional method of compressing and restoring the image data does not use the mechanism that the picture drawn on the printed matter is composed of the dots. The conventional method of compressing and restoring the image data does not compress the picture drawn on the printed effectively.
It is desired to provide a method of effectively compressing the image data generated from the picture composed of the dots and restoring it.
Moreover, in the conventional method of compressing and restoring image data, the dot is compressed and restored without any discrimination from other image elements included in the picture. The dot itself is not restored. Here, if the restored image data is enlarged or contracted, an area ratio of the dot to the entire picture, namely, a dot percent is not stored. For this reason, if the restored image data is enlarged or contracted, a color may be deteriorated. Moreover, if the restored image data is enlarged or contracted, moiré may be induced.
It is desired that the deterioration in the picture quality is not induced even if the operation such as the enlargement or the contraction is performed on the restored image data while the image data generated from the picture composed of the dots is compressed and restored.
Also, in the image data compressing method, the compression of image data is desired to be executed at a high speed. In particular, the compression of image data indicative of a picture whose edge is emphasized is desired to be done at a high speed.
Also, in the image data compressing method, it is desired to provide a compressing and restoring method of image data, which effectively compresses and restores image data generated from a printed matter composed of micro point, each of which does not have the arrangements defined by the print rule and has small areas.